Functional electrical stimulation (FES) is a method for replacing function, such as walking or grasping, that is partially or completely lost after various lesions to the central nervous system, such as stroke or spinal cord injury.
One of the most common deficits in the lower leg is "foot drop". In this condition, the patient is unable to properly activate the muscles that flex the ankle, to enable the foot to clear the ground during the swing phase of walking. Instead, the foot drops and drags along the ground.
Liberson et al, in a paper entitled "Functional Electrotherapy, Stimulation of the Peroneal Nerve Synchronized with the Swing Phase of the Gait of Hemiplegic Patients" Arch. Phys. Med., 1961, 42, 101-105, first proposed using electrical stimulation to correct foot drop. The authors suggested that the stimulation be controlled by a heel sensor. When the person leaned forward and reduced heel pressure, switch contacts built into the shoe heel would open. Upon opening the switch, current would flow to electrodes positioned over the common peroneal nerve near the knee. The nerve would be stimulated and innervate muscles that flex the ankle. Upon muscle contraction, the foot would flex and clear the ground without dragging during swing. When the heel returned to the ground at the beginning of stance phase, the switch contacts would close and stimulation would be terminated.
A number of variants of the basic system have been tried over the years and some success has been attained. The commonly known prior art electronic stimulators are reviewed in an article by Kralj et al, "Enhancement of Hemiplegic Patient Rehabilitation by Means of Functional Electrical Stimulation; Prosthet, Orthot. Int., 1993, 17, 107-114.
However, the most common device fitted for people with the condition of foot drop is still an ankle-foot orthosis ("AFO"). An AFO does not involve electrical stimulation. It is a plastic brace that fits around the lower leg and holds the foot fixed, usually at an angle near 90.degree. with respect to the long axis of the leg.
Although AFO's have several disadvantages, the electrical stimulators have not significantly replaced them because the latter have a number of drawbacks. Included among these drawbacks are:
1. that the electrical stimulators comprise a number of pieces, all of which have to be placed and connected together accurately. Carrying this out is a difficult and time-consuming chore for a patient who has likely lost some manual dexterity and who may have suffered some cognitive impairment as a result of stroke or injury. More specifically, these stimulator units typically comprise: (a) a box containing the stimulator, battery and control electronics, which have to be placed on the body or in a pocket; (b) electrodes which has to be placed accurately on the skin; (c) some garment, such as an elastic knee stocking, for supporting the electrodes in place; (d) a foot switch; and (e) wiring connecting the electronics and electrodes; PA1 2. that the lengthy wiring is prone to breakage; PA1 3. that the foot switch may not work reliably on all of several different surfaces such as pavement, carpet, sand, up and down ramps and the like; and PA1 4. that some patients may not land on their heel or put little weight on it, due to spasticity or contractures in the calf muscles. These problems can lead to failure to trigger the stimulation. An alternative is to have a fixed duration of stimulation. But this may be appropriate at some speeds of walking or levels of fatigue, but not at others. PA1 a band of stretchable, breathable fabric having fastening means at its ends for securement of the band around the user's leg in the form of a ring; PA1 anode and cathode electrodes carried by the band in a stationary or fixed condition; PA1 locating means, carried by the band, for cooperating with the bony protuberance of the tibia, to accurately and reproducibly locate the electrodes over the nerve to be stimulated; PA1 tilt sensor means, carried by the band, for measuring the angle of the lower segment of the leg relative to vertical in the sagittal (forward and back) plane and emitting sensors signals indicative thereof; PA1 battery means, carried by the band, for supplying electrical current, as required; and PA1 control means, carried by the band and connected with the sensor means, electrodes and battery means, for receiving the sensor signals, processing the sensor signals to establish values indicative of the changing angularity of the limb (that is, position of the lower leg with respect to vertical) and to compare the established values with predetermined adjustable ON and OFF threshold values and initiating and terminating the emission of electrical pulses though the electrodes on respectively reaching the ON and OFF threshold values to stimulate the leg muscles in response to phases of body movement.
With this background in mind, I set out to design an electronic stimulator which is easy to don and doff, which automatically and accurately positions the electrodes, which incorporates sensors that more completely monitor body motion and do not require heel pressure, which uses relatively short connecting wires, and which controls stimulation in response to the stage of body motion.
In developing the invention, I have used a locating means disclosed in U.S. Pat. No. 4,431,368, issued to Russek. This patent teaches a garment carrying a member which closely conforms to the protuberance of the spine. By positioning the member over the spine, electrodes carried by the garment are automatically located accurately over nerves to be stimulated.